The present disclosure relates to a post-processing device performing post-processes to a sheet outputted from an image forming apparatus or the like, and an image forming system including this post-processing device.
A post-processing device is connected to an image forming apparatus, such as a copying machine or a printer, to carry-in a sheet outputted from the image forming apparatus after an image is formed on the sheet in the image forming apparatus and to perform post-processes, such as punching process, stapling process and others, to the sheet. The post-processing device temporarily accumulates the carried-in sheet on a process tray and aligns end parts of a predetermined number of the sheets by the process tray, and then, performs the post-processes, such as the stapling process, to these sheets and ejects the post-processed sheet onto a stack tray. There is also a post-processing device lowering the stack tray as the sheet is ejected onto the stack tray.
The post-processing device lowers the stack tray in accordance with the ejection of the sheet, not only when ejecting every the predetermined number of the sheets placed on the process tray to the stack tray, but also when ejecting every one sheet to the stack tray. Thereby, the sheet ejected from an ejection port of the post-processing device is stacked at a top face position of the stack tray or at an uppermost position of the sheets stacked on the stack tray. The stack tray is raised and returned to an original position when all the sheets on the stack tray are taken out.
However, for example, a time required for raising the stack tray from a lower side to an upper side in its movement path is lengthened compared with a time required for performing the post-processes of one sheet. In the above-mentioned post-processing device, in the middle of the post-processes of many sheets, if the entirety or a part of the sheets stacked on the stack tray are/is taken out, the following post-processed sheet is ejected even through the stack tray is not returned to the original position, i.e. located at the lower side. At this time, because the sheet is ejected in the air above the top face position of the stack tray or the uppermost position of the sheets stacked on the stack tray, the sheet is not suitably stacked and a trouble, such as dropping out from the stack tray, occurs.
By contrast, there is a finisher as the post-processing device temporarily accumulating a plurality of sheets outputted from an image forming apparatus on a post-process tray part, performing post-processes to a sheaf of the sheets, and stacking the post-processed sheet sheaf onto a stack tray part (a stack tray) lowering in accordance with carrying-in or stacking of the sheet sheaf. The finisher includes a sheet sheaf stack detecting sensor detecting the sheet on the stack tray part and an elevating function raising the stack tray part to an original position when the sheet sheaf stack detecting sensor detects that the sheet is taken out from the stack tray part. The finisher makes the sheet sheaf wait in the middle of conveyance to the stack tray part until the stack tray part is returned back to the original position after the sheet sheaf on the stack tray part is taken out. The finisher also resumes the conveyance of the sheet sheaf after the return of the stack tray part to the original position.
However, in the post-processing device as the above-mentioned finisher, the sheet sheaf stack detecting sensor detects whether or not the sheet is placed on the stack tray. According to this, when a part of the plurality of the sheets on the stack tray is taken out, the sheet sheaf stack detecting sensor cannot detect the taking-out of the part of the sheets. Therefore, in such a case, because the elevating function does not raise the stack tray, it is impossible to suitably stack the post-processed sheets on the stack tray.
In the above-mentioned post-processing device, when the entire sheets on the stack tray are taken out in the middle of the post-processes of many sheets, it is necessary to stop outputting the sheet from the image forming apparatus and to stop the post-processes of the post-processing device until the stack tray located at the lower side is raised to the original position. The stopping of works, such as the post processes, during the raising time of the stack tray causes waste of working time.